Harbor seal populations and their contributions to fecal coliform pollution in Quilcene Bay, Washington

High fecal coliform concentrations in the northern part of Quilcene Bay have recently been reported and have resulted in the decertification of this area for commercial shellfish growing. The bay also supports a growing population of harbor seals. This study was undertaken to evaluate the potential contribution of harbor seals to the fecal coliform pollution and make recommendations on how to more precisely determine the portion of fecal coliforms contributed by seals. A maximum of 230 harbor seals were counted in Quilcene Bay, primarily concentrated at a log dump on the eastern side of the bay. Seals were present year-round, with no dramatic seasonal patterns in the number of animals. Seal defecation rates were estimated as between 250 to 500 grams per day for a 50 kg seal, with 375 grams the best estimate of average daily fecal production. A variety of bacterial species were identified in 10 fresh seal feces collected from the Dosewallips River Delta (adjacent to Quilcene Bay). Bacillus sp. and E. coli were the most often found predominant bacterial populations. no Salmonella or Yersinia were found in enrichment procedures conducted for these two pathogenic bacteria. Fecal coliform concentrations in the 10 harbor seal feces analyzed by Biochem Environmental Services ranged from 4.0×10⁶ to 9.2×10⁸ coliforms per gram with a geometric mean of 3.1×10⁷. These concentrations are similar to those reported for humans but are higher than for most domestic animals. Ratios of fecal coliforms to fecal streptococcus were much higher than reported for humans or domestic animals. This ratio and some other unique bacterial indicators may be of value in tracing the source of bacteria found in Quilcene Bay. This study indicated that fecal coliform densities in harbor seals are fairly high and given the population size of seals and their defecation rates, seals have the potential to be significant contributors to the high fecal coliform levels in Quilcene Bay.

Recommendations to more accurately determine the proportion of fecal coliform contributed by seals include; 1) increase sample size of seal feces tested, 2) determine portion that dissolves in marine water, 3) determine the portion of seal feces excreted in different parts of Quilcene Bay, 4) use specific bacterial indicators to trace the source of fecal coliforms found in Quilcene Bay waters, 5) test fecal coliform levels at other sites where seals are the only potential source, 6) examine fecal coliform concentrations produced by captive seals.

Citation:

Calambokidis, J. and B. McLaughlin. 1987. Harbor seal populations and their contributions to fecal coliform pollution in Quilcene Bay, Washington. Final report for contract from Jefferson County and the Washington Department of Ecology, Olympia, Washington. 29pp.